1. Field of the Invention
This invention relates to dual-mode electro-optic (EO) sensors that process both active guidance radiation (e.g. laser radiation from a SAL designator) and passive imaging radiation (e.g. emitted or reflected IR) to provide guidance signals, and more particularly to a dual-mode EO sensor that uses the active guidance radiation from target designation as a guide star for wavefront error estimation of the primary optical element without interfering with the normal operation of the EO sensor. The wavefront error estimate may be used to control actuators to compensate a deformable primary optical element to reduce wavefront errors or to improve an estimate of target position. The estimate may be output and used to redesign the primary optical element.
2. Description of the Related Art
Many guided munitions (e.g. self-propelled missiles or rockets, gun-launched projectiles or aerial bombs) use a dual-mode EO sensor to guide the munition to its target. In a semi-active laser (SAL) mode, the sensor detects active guidance radiation in the form of laser radiation from a SAL designator that is reflected off of the target and locks onto the laser spot to provide line-of-sight (LOS) error estimates. In a passive imaging mode, the sensor detects IR radiation emitted from or reflected off of the target. The sources of IR energy are not artificial; they typically follow the laws of Planck radiation. The source may be the blackbody radiation emitted by the target directly or may, for example, be sunlight that is reflected off of the target. The passive imaging mode may be used to provide LOS error estimates to track the target when SAL designation is not available and may be used at the end of flight to process a more highly resolved image to choose a particular aimpoint on the target or to determine whether or not the target is of interest. The passive imaging mode operates at a much higher spatial resolution than the SAL mode.
A dual-mode sensor comprises a primary optical element having a common aperture for collecting and focusing SAL laser radiation and passive imaging radiation along a common optical path. A secondary optical element separates the SAL laser and passive imaging radiation by spectral band and directs the SAL laser radiation along a first optical path to a SAL detector and directs the passive imaging radiation along a second optical path to an IR imaging detector. The optics spatially encode an angle of incidence of the SAL laser radiation (e.g. a laser spot) at an entrance pupil onto the SAL detector. A quad-cell photodiode provides sufficient resolution to determine the LOS error estimate. The passive imaging radiation from a typical target is at long range, such that the EM wavefront at the sensor is considered to be composed of planar wavefronts. The structure of the target is imprinted on the composite wavefront as a summation of planar wavefronts with different slopes. The optics convert these slopes to spatial offsets in the image plane to form an image of the target on the IR detector.
Ideally the optics convert the incident wavefronts into spherical wavefronts that collapse onto the image plane of the optical system. Given an ideal point source positioned on the optical axis, any deviation from the perfect spherical wavefront (i.e. local slope differences of the wavefront) represents a wavefront error that distorts the image in some way and degrades system performance. These wavefront errors may degrade the high-resolution IR mode performance substantially, while having minimal impact on the much lower resolution SAL mode. Sources of error during assembly and manufacturing can include surface shape defects in the primary and secondary optical elements and mechanical stresses on the optical elements from mounting the EO detector or other components.
During production, an interferometer or Shack-Hartman wavefront sensor may be used to measure a wavefront error estimate to qualify the sensor. The wavefront measurement may also be used to directly compensate the errors via a deformable mirror in some applications. Both the hardware and operation of the interferometer and Shack-Hartman wavefront sensor are expensive. Both require an external EO detector as part of the hardware package. Both require an experienced engineer to perform the test. Neither is suitable for testing in the field.
Once put into the field, the guided munition may be susceptible to different thermal loading conditions that distort the optics, causing wavefront errors. A first order thermal loading is caused by equilibrium thermal conditions that deviate from the production line. For example, a guided munition stored in a launch canister in a desert may be subjected to extreme heat whereas a guided munition on-board an aircraft at high altitudes may be subjected to extreme cold. A second order thermal loading is caused by transient aerodynamic heating once the munition has been launched. Given the typical ratio of sizes between the primary and secondary optical components, the thermal loading effects on the secondary optical elements are typically minimal. This means that in most sensors, the greatest source of distortion is the thermal loading of the primary optical element (e.g. a reflective mirror or lens).
The state-of-the-art to addressing the thermal loading effects is to design the primary optical element to handle a wide range of thermal loading conditions. The primary optical element becomes bigger, heavier and more expensive and the opto-mechanical mounting mechanisms more complex to athermalize the design as much as possible.